The push and pull of abandoned channels: how floodplain processes and healing affect avulsion dynamics and alluvial landscape evolution in foreland basins

Abstract

Abstract. River avulsions are an important mechanism by which sediment is routed and emplaced in foreland basins. However, because avulsions occur infrequently, we lack observational data that might inform where, when, and why avulsions occur and these issues are instead often investigated by rule-based numerical models. These models have historically simplified or neglected the effects of abandoned channels on avulsion dynamics, even though fluvial megafans in foreland basins are characteristically covered in abandoned channels. Here, we investigate the pervasiveness of abandoned channels on modern fluvial megafan surfaces. Then, we present a physically based cellular model that parameterizes interactions between a single avulsing river and abandoned channels in a foreland basin setting. We investigate how abandoned channels affect avulsion setup, pathfinding, and landscape evolution. We demonstrate and discuss how the processes of abandoned channel inheritance and transient knickpoint propagation post-avulsion serve to shortcut the time necessary to set up successive avulsions. Then, we address the idea that abandoned channels can both repel and attract future pathfinding flows under different conditions. By measuring the distance between the mountain front and each avulsion over long (106 to 107 years) timescales, we show that increasing abandoned channel repulsion serves to push avulsions farther from the mountain front, while increasing attraction pulls avulsions proximally. Abandoned channels do not persist forever, and we test possible channel healing scenarios (deposition-only, erosion-only, and far-field-directed) and show that only the final scenario achieves dynamic equilibrium without completely filling accommodation space. We also observe megafan growth occurring via ∼100 000-year cycles of lobe switching but only in our runs that employ deposition-only or erosion-only healing modes. Finally, we highlight opportunities for future field work and remote sensing efforts to inform our understanding of the role that floodplain topography, including abandoned channels, plays on avulsion dynamics.

The 1870s Saskatchewan River avulsion: Ice jam or open water flood? A probabilistic approach for cold climate river avulsions

“…Gradient advantages can form over time through sediment deposition on floodplains when a river overtops its banks, resulting in natural levees and/or perched channels. The setup period for this super‐elevation or gradient advantage to develop can serve as a lower limit for the time between avulsions (Martin & Edmonds, 2022). For the Old Channel, millennium‐scale levee sedimentation rates were estimated at 3.63–3.93 mm/year (Morozova & Smith, 2003), which corresponds to a deposition rate of approximately one channel depth per 1,500 years in the vicinity of the channel bifurcation.…”

Morphodynamic Preservation of Fluvial Channel Belts

“…It is not clear whether preservation in alluvial-ridge basins must occur within a coastal backwater zone. Sedimentation patterns in the backwater zone favor aggradation and ridge development rather than lateral migration (Hudson & Kesel, 2000;Mason & Mohrig, 2018;Smith et al, 2020), but fluvial megafans develop similar alluvial-ridge topography without a downstream water body (Martin & Edmonds, 2022). That is, the routing of channel belts into alluvial-ridge basins may not be rare, but quite common across alluvial settings.…”

A Generalized Area‐Based Framework to Quantify River Mobility From Remotely Sensed Imagery

“…Cellular avulsion models are driven by the super-elevation of channel-belts and topographic differences across the fan surface (Jerolmack & Mohrig, 2007). We used an implementation of a fan-scale avulsion model (H. K. Martin & Edmonds, 2022), which was designed to incorporate variable channel-belt reoccupation dynamics and variable avulsion trigger frequencies. We ran the model using different avulsion periods, T A ∈ [5, 10, 50, 100, 150, 200, 300, 500] yr, where higher T A indicates less frequent frequencies (see Movie S3).…”

“…Code for the meandering centerline model is found at https://github.com/zsylvester/meanderpy (Sylvester et al., 2019). Code for the avulsion model is found at https://doi.org/10.5194/esurf-10-555-2022 (H. K Martin & Edmonds, 2022). To access all the underlying data used in this publication, visit https://doi.org/10.25349/D90G71.…”

“…For example, meandering rivers feature bank‐scale migration (Hickin & Nanson, 1984; Mason & Mohrig, 2019) and reach‐scale channel‐bend cutoff (Ashmore, 2013; Constantine & Dunne, 2008; Hooke, 2004), braided rivers evolve through bank‐scale downstream bar accretion and reach‐scale thread abandonment (Ashmore, 2013), and river delta channels periodically avulse at the floodplain‐scale (Ashmore, 2013; Brooke et al., 2022; Ganti et al., 2014; Jerolmack & Swenson, 2007). Previous work has investigated mobility processes in different planform environments using numerical models (Chadwick et al., 2019, 2020; Ielpi et al., 2023; Jerolmack & Mohrig, 2007; H. K. Martin & Edmonds, 2022; Murray & Paola, 1997; Williams et al., 2016), physical experiments (Bryant et al., 1995; Chadwick et al., 2022; Ganti et al., 2016a, 2016b; Limaye, 2020; van de Lageweg et al., 2014), and detailed site‐specific fieldwork (Dietrich & Smith, 1983; Micheli & Larsen, 2011).…”